Further to my post above,
@AuneAudio advised a long press and hold of the Power button for restart (same as some people in this thread have said). If you ever need to do this because the device is unresponsive, pls. bear in mind it may take several attempts of holding Power button for 10s-15s before the device finally (suddenly) starts and responds. That is what happened with this DAP in any case, where after being totally unresponsive over 4-5 attempts, it suddenly started and now it seems to be working properly, with both my partner's SD card and my own SD card. It's a relief!
N.B. Anybody can advise how to update FW on this device (e.g., which file extension is the update in and what steps to take from the menu)? Help would be much appreciated.